As wireless communication has developed, voice communication centered mobile terminals have provided more functions for communicating digital character information. Therefore, a keypad originally designed for telephone number input becomes to include means for inputting characters. The size of a keypad used for a mobile terminal tends to be smaller, thereby limiting the number of buttons on the keypad. Meanwhile, there are 27 alphabet characters in Korean, 24 in English, 46 in Japanese, 24 and more in alphabets derived from the Roman alphabet. Like this, the alphabets of almost every language include over 12 characters that is the number of buttons used on a usual telephone keypad. Therefore, one button or the combination of two or more buttons is used to input one character through the telephone keypad.
Conventional methods of inputting alphabet characters using a small keypad can be classified into a character selection type, a character creation type, and a mixed type. In the character selection type, a plurality of characters are assigned to a single keypad button, so a character (hereinafter, referred to as a “target character”) which a user intends to select on a keypad can be selected by pressing a corresponding button a predetermined number of times. This type of input method is used for English as proposed by the E.161 recommendations of the International Telecommunications Union—Telecommunication Standardization Sector (ITU). In the character creation type, primary or atomic character elements (stokes such as →, ↓, , , ⊃, ◯, etc) are individually assigned to keypad buttons, so a target character is created by operating the buttons so that the shape of the target character can be formed. In the mixed type, some of the alphabet characters are processed according to the character selection type, and the remaining characters are processed according to the character creation type. In the example of the mixed type for the Korean alphabet, consonants are processed according to the selection type and vowels are processed according to the character creation type.
As described above, in the conventional three types, a target character is input by operating one or more buttons on a keypad.
The character selection type can be divided into several methods as follows. In a first method, a plurality of characters (generally, 3˜4 characters) are assigned to each button, so a character assigned first to the button is selected when the button is pressed once, a character assigned secondly to the button is selected when the button is pressed two times and so on. In a second method, characters (generally, 3˜4 characters) are assigned to each button, so a character assigned first to the button is selected when the button is pressed once, and a character assigned secondly to the button is selected by the combination of the button and a particular button (such as [#] or [*] button). In a third method, characters are assigned between two buttons, and a target character is selected by the combination of the two buttons.
There are some innate faults in the first method. If it is assumed that three characters ‘A’, ‘B’ and ‘C’ are assigned to a [2] button, when a user wants to sequentially input the characters ‘A’, ‘B’ and ‘C’, the user is supposed to consecutively press the [2] button six times. In this case, there is ambiguity among the input of ‘A’, ‘B’ and ‘C’, the input of ‘B’, ‘B’ and ‘B’ and the input of ‘C’, ‘B’ and ‘A’. To overcome this problem, the user is supposed to press the button six times with time delay between the characters like [2]-[2][2]-[2][2][2]. However, it is not easy to verify that a target character is input correctly in a terminal such as a normal telephone unprovided with a display window. Moreover, according to the first method, the more characters that are assigned to each button, the greater the number of times a button needs to be pressed for one character. For example, when three alphabet characters are assigned to each button for English, it is necessary to press buttons an average of two times for each character. When five alphabet characters are assigned to each button for Japanese, it is necessary to press buttons an average of three times for each character. Accordingly, when 5 or more characters are assigned to each button for an alphabet, it may be necessary to press buttons 5 or more times for one character, so it is very inconvenient, and accuracy is low.
In the second method, when a user wants to input a character not assigned first to a button, the user should press a particular button such as a [#] or [*] button in addition to the button to which the character is assigned. Accordingly, it is not easy for the user to be familiar with a character input method.
The third method has the advantages of simplicity and consistency in inputting characters. However, this method is not fit for a mnemonic number dialing plan, and it is not easy to print characters in the small space between buttons on a miniaturized mobile terminal such as a cellular phone, a PCS phone, and an IMT-2000 phone.